Memory devices are electronic devices arranged to store electrical signals. For example, a basic memory element may be a fuse that can either be open or be closed. Open and closed states of the fuse may be used to designate one bit of information corresponding to a value of 1 or 0. A plurality of memory elements can be combined in various arrangements in order to store multiple bits arranged in words or other combinations. Various electronic circuits including semiconductor devices such as transistors are used as memory elements.
Memory elements may be classified in two main categories: volatile and nonvolatile. Volatile memory loses any data as soon as the system power is turned off. Thus, it requires constant power to remain viable. Most types of random access memory (RAM) fall into this category. Non-volatile memory does not lose its data when the system or device is turned off. An NVM device may be implemented as a MOS transistor that has a source, a drain, an access or a control gate, and a floating gate. It is structurally the same as standard MOSFET except its gate is floating, which is electrically isolated.” There are also many structural variations of MOS devices to enhance the operation or reduce the size of NVM devices.
A range of considerations including a purpose of the device, power consumption, size, retention capacity and duration may influence design of non-volatile memory devices. For example, some NVM devices may be categorized as floating gate or charge-trapping from a programming perspective.
Non-volatile memory devices may also be implemented as NVM arrays that include a plurality of NVM cells arranged in rows and columns. In general, single-transistor n-channel NVM cells operate as follows. During an erase operation, electrons are removed from a floating gate of the NVM cell, thereby lowering the threshold voltage of the NVM cell. During a program operation, electrons are inserted into the floating gate of the NVM cell, thereby raising the threshold voltage of the NVM cell. Thus, during program and erase operations, the threshold voltages of selected NVM cells are changed. During a read operation, read voltages are applied to selected NVM cells. In response, read currents flow through these selected NVM cells.
Certain types of nonvolatile memory devices, circuits employing floating-gate devices, high-voltage drivers and other circuits fabricated in logic CMOS require relatively high voltages (e.g., 10 volts in a 3.3 volt CMOS process). For example, in nonvolatile memory devices high voltages are often used to by circuits that program and erase information stored on the floating gates, and on-chip charge pumps are generally used to generate these high voltages. In all these circuits, switches are required to selectively apply the high voltages to specific circuit elements.
Tolerating a high-voltage supply, generating intermediate voltages, as well as generating high-voltage drive signals from low-voltage logic-gate-level signals while minimizing power consumption, are substantial design parameters for switch circuits adapted to program memory devices.